This invention relates to nitrification in activated sludge systems at low solids retention times, and more particularly, to use of a second stage activated sludge system in conjunction with another activated sludge system to achieve nitrification of ammonia derived from the influent wastewater using the concept of nitrifier bioaugmentation or seeding.
Discharge of ammonia from municipal wastewater treatment plants (WWTPs) has becomes more stringently regulated over the last two decades. The un-ionized form of ammonia is toxic to aquatic life, and as such, its presence in WWTP effluents is often restricted.
Nitrification is a two-step biologically mediated conversion of ammonia to nitrate and is the most common method of achieving ammonia removal from municipal wastewaters. Nitrifying bacteria have a much lower specific growth rate than the heterotrophic organisms that are responsible for carbonaceous removal and are predominant in the activated sludge process. The specific growth rate of nitrifiers is also strongly dependent on the wastewater characteristics and temperature. Winter wastewater temperatures in northern climates can range from 10xc2x0 C. to 12xc2x0 C. or lower. Relatively high solids retention times (SRTs), typically greater than eight days, are generally required to ensure year round nitrification.
A problem faced by many municipalities is the need to either meet new or more stringent limits for ammonia when plant expansion is required. Plants previously designed to provide only carbonaceous removal require substantial modifications that can include de-rating of existing facilities to meet the new ammonia limits. This results from the fact that nitrifiers, the autotrophic organisms that carry on ammonia oxidation, have a lower maximum specific growth rate compared to the heterotrophic organisms that dominate activated sludge processes. In addition, the nitrifier maximum specific growth rate decreases markedly with temperature (Grady et al., (1999) Biological Wastewater Treatment, Second Edition, Marcel Dekker, N.Y.). Therefore, the solids retention time (SRT) at which a nitrifying system needs to be operated has to be sufficiently long to ensure that an adequate nitrifier population remains in these systems.
Wastewater treatment plants that were designed for removing carbonaceous material often operate at relatively low SRTs (i.e. 5 days or less) and their aeration and secondary clarifier tankage is sized accordingly. At these SRTs, year round nitrification is not normally possible, especially at the low temperatures that are encountered during the winter months in northern climates (e.g. 10xc2x0 C. and 12xc2x0 C.). Increasing the SRT will result in an increase in the mixed liquor suspended solids (MLSS) concentrations, and frequently the existing secondary clarifiers do not have the capacity to operate at these augmented MLSS concentrations and solids loadings. The traditional approach to allow such plants to provide nitrification is through the construction of additional aeration and secondary clarifiers. As such, significant capital expenditure is typically required to provide for year round nitrification and this has lead to the investigation of different strategies that allow this upgrade to be achieved in a more cost effective manner.
A common theme of many of these strategies is the concept of nitrifier bioaugmentation, which makes use of a separate seed source of nitrifiers fed to a low SRT reactor to support nitrification (Constantine, Bioaugmentation to Achieve Nitrification in Activated Sludge Systems, Masters Thesis, McMaster University 1996). One of the earlier manifestations of bioaugmentation was in certain trickling filters/activated sludge processes, where partial nitrification takes place in the trickling filter (Daigger, et al. (1993). Process and Kinetic Analysis of Nitrification in Coupled Trickling Filter/Activated Sludge Processes. Water Environment Research, Vol. 65, pp. 679-685.). Nitrifiers growing in the trickling filter will slough off and enter the activated sludge process. This is an example of bioaugmentation, where nitrifiers growing in the trickling filter will seed the activated sludge process, allowing its operation at a decreased SRT, while still providing stable nitrification.
Another example of nitrifier bioaugmentation is given by the work of Constantine (1996). Two parallel sequencing batch reactors (SBR) were operated at two different SRTs. One of the SBRs was operated at an elevated SRT, so that nitrification would always occur in this system, and this reactor was referred to as the xe2x80x9cDonorxe2x80x9d reactor. The other SBR was operated at a low SRT that would not allow nitrification to proceed, and this reactor was referred to as the xe2x80x9cReceiverxe2x80x9d reactor. Waste activated sludge (WAS) was directed from the Donor to the Receiver reactor, resulting in a constant supply of nitrifiers to the Receiver reactor. This allowed stable nitrification to take place in the Receiver reactor, at SRTs that would not normally allow nitrification to proceed were it not for bioaugmentation. The major drawback of this approach is that along with nitrifiers, a significant amount of non-nitrifier material is associated with the WAS, leading to a significant impact on the MLSS concentration of the Receiver reactor.
Bioaugmentation was also described in a two-stage process by Tendaj-Xavier, 1983 (Tendaj-Xavier (1983). Biological Treatment of Sludge Water from from Centrifugation of Digested Sludge, Dissertation, Royal Technical University). The dissertation generally suggests growing nitrification bacteria on a portion of the wastewater stream and seeding the nitrified bacteria into the remaining stream. The Dissertation reports that the process arrangement requires a high investment cost. Also, the process could not be used at the desired facility due to the lack of space required for the process configuration that grows the nitrifiers.
The process reported in Kos (Kos, P. (1998). Short SRT (Solids Retention Time) Nitrification Process/Flowsheet Wat. Sci. Tech., Vol. 38, No. 1, pp. 23-29.), and described in U.S. Pat. No. 5,811,009, also relies on the concept of bioaugmentation and this process configuration mitigates some of the drawbacks of the process proposed by Constantine (1996). In this case, a sidestream reactor, distinct from the mainstream, treats the recycle stream from anaerobic digesters (e.g., supernatant or dewatering centrate). These recycle streams are rich in ammonia, which is released during anaerobic digestion, thereby allowing the generation of an enriched culture of nitrifiers in this sidestream reactor. An additional benefit is that the temperature of these recycle streams is typically high, which is favorable for nitrification. Kos (1998) demonstrated, through steady-state simulation, that this application of nitrifier bioaugmentation allowed nitrification to proceed in the mainstream process at reduced SRTs. Therefore, the process reported in Kos (1998) would require somewhat less secondary treatment tankage compared to a traditional nitrification system.
This process configuration suffers from a number of potential problems especially with respect to the sidestream plant operation, including the potential of high supplementary alkalinity requirements to maintain process stability; possible process instability associated with substrate and product inhibition as described in Anthonisen, et al. (1976) Inhibition of nitrification by ammonia and nitrous acid, Journal of the Water Pollution Control Federation, Vol. 48, pp. 835-852. The system also has the potential of poor SRT maintenance, as enriched nitrifier cultures are known to possess poor settling characteristics (U.S. Environmental Protection Agency, (1993). Process Design Manual for Nitrogen Control, EPA/625/R-93/010, U.S. Environmental Protection Agency, Cincinnati, Ohio.). Additionally, the amount of nitrifiers formed in the mainstream is largely a function of the dose of nitrifiers from the sidestream. The process also results in an extremely high mass of total mixed liquor suspended solids under aeration. Another concern is whether the process configuration proposed by Kos (1998) is as effective under dynamic conditions (such as changes in the normal diurnal variation in wastewater flow and load) as it is under steady-state operation.
Accordingly, there remains a need for a new process configuration to allow year round nitrification, while maintaining the relatively low SRTs typical of conventional activated sludge (CAS). Also needed is a process configuration minimizing the quantity of solids under aeration. Further needed is a process configuration readily adaptable to existing treatment plants requiring a minimum amount of space to install. A process configuration operable at relatively low temperatures and requiring a minimum amount of supplemental chemicals is also highly desirable.
The present invention is intended to overcome one or more of the problems discussed above.
Various publications and patents have been referred to herein. These publications are incorporated by reference herein in their entirety.
The present invention provides a wastewater treatment process providing nitrification comprising subjecting a first stream of influent having an ammonia concentration to a first BOD removal treatment process to yield a first effluent, subjecting a second stream of influent to a second activated sludge process, subjecting the first effluent to a second stage reactor capable of growing nitrifiers and nitrifying the first effluent to generate nitrifier-enriched effluent, and adding a portion of this nitrifier-enriched effluent to the second stream of influent to promote nitrification in the second stream activated sludge process.
The present invention also provides a wastewater treatment plant comprising a first BOD removal treatment process receiving a first stream of wastewater influent and emitting an effluent, a second stream activated sludge process receiving a second stream of influent wastewater, a second stage nitrification reactor receiving the effluent from the first BOD removal treatment process, the second stage nitrification reactor operating at an SRT sufficient to provide essentially complete nitrification and to grow nitrifiers, and means for conveying a portion of the biomass produced, containing nitrifiers, from the second stage nitrification reactor to the second stream activated sludge process to provide nitrification in the second stream activated sludge process.
The present invention further provides a method for modifying an activated sludge process configuration to enhance ammonia nitrification, comprising providing an existing wastewater treatment plant comprising a first activated sludge process and a second activated sludge process, providing a second stage reactor for nitrification and to grow nitrifiers in fluid communication with an effluent from the first activated sludge process, and providing a means for directing a portion of the biomass produced from the second stage reactor to the second activated sludge process such as a closed conduit or open channel.
The present invention overcomes many of the disadvantages of prior treatment processes including Kos (1998). Among the most significant advantages of the present invention are use of a mainstream process to provide supplemental nitrifiers which also treats a significant percentage of the influent wastewater flow and discharges this treated flow as final effluent. This makes the system adaptable to existing treatment plants without significantly degrading throughput. Use of the preferred membrane bioreactor also minimizes the space necessary to implement the process, further making it useful for retrofitting existing plants. The ability of the process configuration to operate at low temperatures allows for its use in colder climates. Minimizing the solids subject to aeration by producing nitrifiers from a clarified secondary effluent further improves plant efficiency. In addition, in almost all applications the process configuration requires no use of supplemental chemicals.